The present invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device which is called In-Plane Switching mode.
Liquid crystal display devices are widely used in various kinds of electronic equipment such as display devices such as display monitors for personal computers and television receivers. Various types of liquid crystal display devices are known. A liquid crystal display device of the type which is called In-Plane Switching (IPS) mode has a liquid crystal panel generally made of two substrates disposed in opposition to each other with a liquid crystal interposed therebetween. In each pixel area on the surface of either one of the substrates that is in contact with the liquid crystal, a pixel electrode is formed and, in addition, a counter electrode is formed at a location close to the pixel electrode. This type of liquid crystal display device generates an electric field (lateral electric field) parallel to the surface of the substrate between the pixel electrode and the counter electrode, thereby controlling the alignment direction of the liquid crystal in the plane between the surfaces of the substrates.
In other words, the In-Plane Switching mode of liquid crystal display device is constructed to control the amount of transmission of light through the area between the pixel electrode and the counter electrode by means of the alignment direction of the liquid crystal to which the electric field is applied. Although a module which includes a liquid crystal panel as well as constituent elements such as a driver circuit and an illuminating light source is generally called a liquid crystal display device, the term “liquid crystal display device” to be used herein collectively indicates both a liquid crystal panel and a liquid crystal display device.
It is known that such a liquid crystal display device is superior in a so-called wide viewing angle characteristic in that the state of its display does not vary even if its display screen is observed in oblique directions.
Each of the pixel electrode and the counter electrode has so far been formed of a conductive layer which does not allow light to be transmitted therethrough.
It has recently been known, however, that one electrode made of a transparent electrode material is formed over the entire area except the periphery of a pixel area, while the other electrode made of a stripe-shaped or rectangular transparent electrode is formed over the one electrode with an insulating film interposed therebetween. Since the transparent electrodes are used for the pixel-driving electrodes, a so-called aperture ratio is greatly increased.
The above-described type of art is disclosed, for example, in SID (Society for Information Display) 99 DIGEST: PP. 202-205 and Japanese Patent Laid-Open No. 202356/1989.
In a so-called large-sized liquid crystal display device having an orthogonal length greater than or equal to 46 cm (nominal 18 inches) or 51 cm (nominal 20 inches), it has been required to lower the resistance of counter lines or voltage application lines (gate lines and drain lines) for switching elements such as thin film transistors TFT.
To meet the demand for lower resistances, aluminum or an alloy which essentially contains aluminum (hereinafter referred to simply as aluminum) is suitably used as the material of such interconnection lines.
In addition, to improve the luminance of the screen of the liquid crystal display device, it is desirable that pixel electrodes and counter electrodes be made of a transparent conductive film (hereinafter referred to as ITO film or the like) such as ITO (Indium Tin Oxide), IZO (Indium Zinc Oxide) or IGO (Indium Germanium Oxide).
In the case where aluminum is used for gate lines, drain lines or counter lines, while ITO or the like is used for pixel electrodes or counter electrodes, it is necessary that ITO or the like which constitutes the pixel electrodes and the counter electrodes be stacked on an aluminum film which constitutes such lines, because of constructional necessity in the formation of electrical connections and a pixel pattern.
However, since aluminum and ITO or the like greatly differ in corrosion potential, there are some cases where during the wet etching of individual patterns for interconnection lines, pixel electrodes or counter electrodes, aluminum is dissolved in a developing solution and ITO or the like is reduced, and transparency is degraded and the transmissivity of pixels is lowered to a great extent.
In addition, there are some cases where if a pattern of ITO or the like is formed after the formation of an aluminum pattern (interconnecting pattern), aluminum is corroded during the etching of ITO or the like and an initial function is lost.
In addition, if an interconnection line is formed of aluminum, it is difficult to electrically connect ITO or the like, which constitutes an oxide transparent conductive layer, with an aluminum film in the state of being in direct contact with the aluminum film. For this reason, if aluminum and ITO or the like are to be brought into electrical contact with each other, a metal film having a small electrical contact resistance with respect to ITO or the like needs to be separately deposited and processed on the aluminum film.